From NUnit 2.x

In general, there are two approaches to running tests built against earlier
versions of the NUnit framework from 2.0 through 2.5.10:

Run the tests without recompilation. This is the best approach when no
further changes are to be made to a project. Ensure that the framework
assembly against which the tests were built continues to be avaialble
in the test directory.

Recompile the tests referencing the latest version of the framework assembly.
This is usually the best option when a project is still under development.

Note:
NUnit 2.6 no longer recognizes "old style" test cases - those
identified by a name beginning with "Test". If you have such tests, you will
need to convert them to use the TestAttribute and recompile your tests. In such cases, it makes sense to recompile against
the latest framework.

From NUnit 1.x

Tests originally compiled against a version of NUnit prior to 2.0 will need
to be rewritten and recompiled in order to run under NUnit 2.6.